Kerfir is not very good IMO. The contents can be too mixed and you never know what you are getting. Fermented foods can be good but I do not rate kerfir because it is too hit or miss.
Vitamin C is fairly poorly absorbed unless you go for liposomal C which is considerably better. You can make fairly decent quality liposomal C. http://www.qualityliposomalc.com/
Vitamin d, make sure it is vitamin d3.
For magnesium you could try mixing ascorbic acid with magnesium carbonate to form magnesium ascorbate which is a magnesium salt with decent absorption but no where near as good as liposomal C. One benefit is that it neutralises the acidity of ascorbic acid because magnesium carbonate is alkaline and reacts in an acid/base reaction and turns from a white liquid to clear when the quantities are correct. Warn but not hot water helps speed up the reaction. There are more absorbable forms of magnesium but they are a lot more expensive. Magnesium carbonate mixed with acids forms fairly well absorbed salts and it quite inexpensive and goes a long way.
Anthocyanins/anthocyanidins/proanthocyandins etc which are the colours present in many plants and are quite high in many berries. The problem with berries such as Cherries, blackcurrants, blueberries and similar is that they are quite high in sugars and can raise the blood glucose which can affect vitamin C usage. Grape Seed Extract would get around that. These all have strengthening effects on the bones, tissues, skin and circulatory system.
Multivitamins can be unhelpful due to using cheap ingredients which can be problematic. A fairly significant percentage of people cannot make use of folic acid which is common in cheap multis. It's worse than this because it's use can prevent the body from using any natural folates that are absorbed in the diet which can mean a functional folate deficiency. This can have very bad effects on the circulatory health in some people as natural folate is an essential nutrient for lowering homocysteine. If that fails then vitamin B6 (P-5-P form is best) is needed for the transsulfuration pathway to lower homocysteine along with serine amino acid but a normal diet should meet that amino acid requirement.
Therefore I would avoid consuming folic acid, I do as I have reason to believe that there is a strong possibility that this could affect me although I have not had a genetic test to confirm it yet. 23andme can do that.
I use Life Extension two per day multi which supplies reasonable amounts of most all the important items. There is a little magnesium oxide which is a bit useless but I forgive this as the price is quite reasonable. Many of the vits are in a mix of enzyme activated and none enzyme activated forms which require some work by the body. There are some of each for each vitamin.